Contractile force in muscle fibers is generated by cross-bridges that form between the myosin-containing and actin-containing filaments and hydrolyze ATP. During shortening, the two types of filaments slide relative to each other. The structural changes associated with cross-bridge formation have been measured by X-ray diffraction techniques, using radiation from both rotating anode generators and synchrotron sources. The diffraction patterns were recorded with both one-dimensional, electronic, position-sensitive, X-ray detectros and film. Muscle preparations were made from rabbits and frogs, and they included single skinned fibers and small bundles of skinned fibers. The diffraction pattern from the cross-bridge that forms in low ionic strength relaxing solution was found to be significantly different from that of the rigor (ATP-free) cross-bridge. This finding shows that actomyosin cross-bridges have two significantly different structures, and it suggests that contractile force in muscle fibers may be generated by a transition between these two actomyosin configurations. The ordering of the myofilaments in relaxed rabbit muscle was found to depend on the electric charge on the filaments. Thus the double hexagonal arrangement of the myofilaments appears to be maintained by a balance between attractive and electrostatic repulsive forces. A laser diffraction technique for adjusting sarcomere length of muscles during tendon transfer operations was used successfully in a clinical setting. The overall goal of the project is to find out how the actomyosin system in muscle fibers produces force and motion, and how the mechanical responses are regulated.